This invention relates to charge dissipation in electrostatically driven devices, such as micro-electro-mechanical systems (“MEMS”) devices.
In a typical MEMS device, a movable structure that includes a conductive member is resiliently mounted to a substrate. Resilient mounting may be effected by, e.g., one or more micromachined springs, a membrane, a cantilever, or a torsional plate. The substrate includes a dielectric material having a surface upon which a plurality of electrodes are disposed. The electrodes disposed on the dielectric surface are spaced apart from each other, so that a gap of dielectric material exists on the surface. Applying a potential difference between the movable conductive member and one or more of the electrodes on the substrate surface produces an attractive electrostatic force urging the movable structure toward the electrodes. The substrate with its electrodes forms an electrostatic structure adapted to drive the movable structure electrostatically. The resilient mounting of the movable structure provides a restoring force.
In certain applications, the deflection of a MEMS device's movable structure ideally would be a unique function of the potentials of each of the fixed electrodes on the substrate surface and the movable conductive member. If it were, the device could be operated in an open-loop manner; application of predetermined constant voltages to the fixed electrodes with respect to the movable conductive member would produce forces on the movable structure causing it to assume a well-defined position and then remain at that position for as long as the predetermined constant voltages continued to be applied.
Real dielectrics are imperfect, and when voltages are applied to the fixed electrodes of a MEMS device, charges may move in or on the dielectric gap that separates the electrodes. Charges can move within the dielectric due to filling or emptying of charge traps in the dielectric, and mobile ions can migrate along the surface of the dielectric. The changing charge distribution resulting from such moving charges causes a time-dependent electrostatic force on the movable structure, whose position thus changes with time (“drifts”) in response to the changing force, even if the voltages applied to the electrodes are held constant. Drift can be a problem in electrostatically driven devices.
One solution to drift problems would be to provide sensors responsive to the position of the movable structure and feedback electronics, and to adjust the voltages applied to the electrodes so as to maintain the movable structure in a desired position. However, such sensors and feedback electronics can be costly and bulky, and can dissipate far more power than the MEMS device.
Another solution to drift problems arising from mobile charges in the dielectric of certain electrostatically driven devices has been to deposit or grow a thin conductive layer, referred to as a charge-dissipation layer, on top of the dielectric to bleed off surface charge from the dielectric. For instance, U.S. Pat. No. 5,949,944 describes such a charge dissipation layer for LiNbO3 modulators, which like MEMS devices are electrostatically driven and may experience problems due to accumulation of surface charges. Forming a charge dissipation layer may require processing steps that might preferably be avoided; moreover, the electrical properties of charge dissipation layers may be difficult to control, and may change due to oxidation or corrosion.